A fuel cell is one kind of electrical generators which take out electric energy by electrochemically oxidizing a fuel such as hydrogen or methanol, and has received attention as a clean energy supply source, in recent years. Particularly, since a polymer electrolyte fuel cell has a low standard working temperature of around 100° C., and has high energy density, wide application as an electrical generator for a distributed electric power generation facility of a relatively small scale or a mobile object such as an automobile or a marine vessel is expected. Further, the fuel cell also receives attention as an electric supply for small movable equipment or portable equipment, and installation into a mobile phone, a personal computer or the like, in place of secondary cells such as a nickel-metal hydride cell and a lithium ion cell, is expected.
In the fuel cell, usually, anode and cathode electrodes in which a reaction for electric power generation occurs, and a polymer electrolyte membrane which is to be a proton conductor between the anode and the cathode constitute a membrane electrode assembly (hereinafter, abbreviated as MEA in some cases), and the fuel cell is composed of a cell as a unit comprising separators and the MEA interposed between the separators. The polymer electrolyte membrane is mainly composed of an ionic-group-containing polymer (polymer electrolyte material), and a polymer electrolyte composition in which an additive or the like is mixed in order to enhance durability can also used.
The polymer electrolyte composition is also suitable for a binder of an electrode catalyst layer or the like which is used in a particularly hostile oxidizing atmosphere. Examples of the required property of the polymer electrolyte membrane and the polymer electrolyte composition include firstly high proton conductivity, and it is necessary that the polymer electrolyte membrane has high proton conductivity particularly even under high temperature and slightly humidified conditions. Further, since the polymer electrolyte membrane and the polymer electrolyte composition bears a function as a barrier which prevents a direct reaction between a fuel and oxygen, the membrane is required to have low permeability of the fuel. In addition, examples of the required properties include chemical stability for enduring a strong oxidizing atmosphere during fuel cell operation, mechanical strength and physical durability which can endure membrane thinning and repeating of swelling and drying, and the like.
Previously, in the polymer electrolyte membrane, Nafion (registered trademark) (manufactured by E. I. du Pont de Nemours & Company) which is a perfluorosulfonic acid polymer has been widely used. There are problems that Nafion is very expensive since Nafion (registered trademark) is produced via multistage synthesis, and that Nafion causes great fuel crossover. In addition, there have been pointed out a problem that mechanical strength and physical durability of a membrane are lost due to swelling and drying, a problem that a softening point is low and use at a high temperature is not possible and, further, a problem of disposal after use, and a problem that recycle of the material is difficult. Further, development of a hydrocarbon electrolyte membrane has been activated in recent years as a polymer electrolyte material, which can replace Nafion (registered trademark), is inexpensive and is excellent in membrane properties.
However, all of these polymer electrolyte membranes have a problem that chemical stability is deficient in the case of being used for the polymer electrolyte fuel cell. While a mechanism of chemical degradation is not adequately clarified, a polymer chain or a side chain is cut due to hydrogen peroxide mainly generated at an electrode during power generation and hydroxy radicals generated when the above-mentioned hydrogen peroxide reacts with an iron ion or copper ion in a membrane, and therefore the polymer electrolyte membrane causes membrane thinning and becomes brittle. In addition to this, there is a problem that while swelling and shrinkage are repeated according to changes in humidity, the polymer electrolyte membrane becoming brittle is damaged leading to difficulty with power generation.
In such a situation, investigations are made for improving chemical stability and durability by use of a polymer electrolyte composition in which an antioxidant is compounded in a perfluoro-type electrolyte membrane or a hydrocarbon-type electrolyte membrane.
For example, Patent Document 1 proposes a polymer electrolyte composition in which cerium ions or manganese ions are compounded in a perfluorosulfonic acid-type polymer or a sulfonic acid group-containing polyether ketone-type polymer.
Further, Patent Document 2 proposes a peroxide decomposition catalyst formed by coordinating a nitrogen atom of imidazole, pyridine or the like to a base metal atom such as manganese or iron.
Moreover, Patent Documents 3 and 4 propose a polymer electrolyte composition in which phenanthroline derivatives or a complex of phenanthroline and a cerium ion or a manganese ion is compounded in a perfluoro-type electrolyte membrane.